1. Field of the Invention
The present invention relates to traffic-information distribution methods and navigation apparatuses. More specifically, the present invention relates to a traffic-information distribution method for distributing traffic information from an information distribution center to a navigation apparatus mounted on a vehicle, and to an on-vehicle navigation apparatus for carrying out the traffic-information distribution method.
2. Description of the Related Art
A navigation apparatus used on a vehicle includes a map-data storage device such as a DVD (digital versatile disk), having map data recorded thereon, a display, and a vehicle-movement detector for detecting a current position and orientation of the vehicle, including a gyro, a GPS (Global Positioning System) receiver, and a velocity sensor. The navigation apparatus reads map data for an area including the current position of the vehicle from the map-data storage device, draws a map image of an area around the current position of the vehicle based on the map data and superimposes a vehicle-position mark (location) on the map image. Furthermore, in accordance with movement of the vehicle, the navigation apparatus scrolls the map image, or moves the vehicle-position mark with the map image remaining fixed on the screen, allowing a user to recognize where the vehicle is currently traveling.
An on-vehicle navigation apparatus is usually capable of route navigation, allowing a user to readily travel to a desired destination. Route navigation is executed in the following manner. A route that links a starting point and a destination with the lowest cost is automatically found based on map data, by a simulation based on the breadth-first search or Dijkstra algorithm. The route that has been found is stored as a navigation route. While the vehicle is traveling, the navigation route is shown on the map screen as distinguished from other roads by color and width. Furthermore, when the vehicle is within a predetermined distance from an intersection where the vehicle should change its direction on the navigation route, an arrow indicating a direction to be taken is shown at the intersection on the map image. In this manner, the user is navigated to the destination.
A cost refers to a value representing the appropriateness as a navigation route, such as a value calculated by multiplying a distance with a constant in accordance with road widths, road types (e.g., ordinary road or highway), right and left turns, etc., or an estimated traveling time of the vehicle. Even if two routes have the same distance, cost may differ depending on whether the user prefers to use a toll road, whether the user places priority on distance or time, etc.
Maps stored in the map-data storage device, such as a DVD, are divided into areas by appropriate longitudinal and latitudinal intervals in accordance with a reduction scale such as 1/12,500, 1/25,000, 1/50,000, or 1/100,000. In the map data, roads and the like are stored each as a group of coordinate values of nodes represented by longitude and latitude. A road consists of a series of two or more nodes, and a path that connects two nodes is called a link. The map data includes (1) a road layer including a list of roads, a node table, and a list of nodes forming intersections; (2) a background layer for showing roads, buildings, parks, rivers, and the like on the map image; and (3) a text/symbol layer for showing text including names of administrative regions, such as city names, town names, and village names, names of roads, names of intersections, and names of buildings, and for showing map symbols and the like.
Recently, systems for providing traffic information from outside vehicles, such as VICS (Vehicle Information and Communication System), have come into practical use. In VICS, various traffic information, such as accident information, congestion information, and parking-vacancy information, is provided using optical beacons, radio beacons, and FM multiplex broadcasting. An on-vehicle navigation apparatus receives the traffic information by a beacon receiver or an FM-multiplex-broadcast receiver, and displays the traffic information on a display. For example, upon receiving traffic information, the navigation apparatus shows a red arrow along a relevant road, indicating congestion, in a map image on the display.
Furthermore, some navigation apparatuses are capable of dynamic route guidance (DRG), i.e., capable of automatically searching for a route for detouring congestion based on congestion information of VICS. This allows a vehicle to travel to a destination while detouring congested roads by following navigation provided by such a navigation apparatus.
However, optical beacons or radio beacons cannot be received without visiting locations where the beacons are provided. As for VICS information provided by FM multiplex broadcasting, reception is allowed within a service area of FM broadcasting. However, since information is updated every five minutes, it usually takes a time on the order of ten minutes for the user to receive VICS information after an FM-multiplex-broadcast receiver starts operation. Thus, generally, a navigation route is searched for without paying consideration to VICS information, and the vehicle starts traveling along the navigation route.
If the navigation apparatus is capable of DRG, the navigation apparatus checks congestion and the like on the navigation route upon obtaining VICS information while the vehicle is traveling, and searches for a detour route if congestion on the navigation route is observed. However, since the vehicle travels some distance before the navigation apparatus obtains VICS information, in some cases, a branch point is passed before VICS information is obtained, thereby failing to avoid the congestion in time.